1. Field of the Invention
This invention relates to an engine control system for controlling an ignition timing and an air-fuel ratio to prevent spark knock of an engine.
2. Description of the Prior Art
In the engine control system disclosed in Japanese Unexamined Patent Publication No. 59(1984)-200042, the air-fuel ratio is controlled by feedback control in a feedback control zone and is controlled by open loop control in an enrich zone. In the enrich zone, the ignition timing is retarded when spark knock occurs. However, when the ignition timing is retarded to prevent spark knock, there arise problems that the engine output power is lowered to adversely affect fuel economy and the exhaust gas temperature is raised. Accordingly, in the engine control system, after the ignition timing is once retarded to eliminate the spark knock, the air-fuel ratio is controlled to advance the ignition timing without occurrence of spark knock. As is well known, engine output power is generally increased for a given operating condition of the engine as the ignition timing is advanced, and spark knock becomes less apt to occur, i.e., a critical ignition timing earlier than that at which spark knock will occur is moved toward the advance side, as the air-fuel ratio becomes richer. That is, in the engine control system, the ignition timing is retarded by a certain crank-angle when spark knock occurs and then is controlled by feedback control to an earliest timing within a range in which knock cannot occur, and then the air-fuel ratio is controlled so that the ignition timing can be approximated to a predetermined optimal value on the basis of comparison of the earliest timing determined by the feedback control and the predetermined optimal value. The predetermined optimal value is generally determined taking into account fuel economy, engine output power, exhaust gas temperature and the like. However, the engine control system disclosed in the patent publication identified above is disadvantageous in that since the predetermined optimal value is fixed irrespective of the fuel octane value, the ignition timing cannot be controlled to an optimal value following change of the fuel octane value. That is, the critical ignition timing, earlier than which spark knock will occur, changes with the air-fuel ratio, and the critical ignition timing for a given air-fuel ratio depends upon the fuel octane value and is moved toward the advance side as the octane number of the fuel increases. Accordingly, for example, if said predetermined optimal value is determined for regular gasoline, and if high octane gasoline is supplied, the ignition timing and the air-fuel ratio will be controlled to values deviated from the optimal values for high octane gasoline. That is, on the knocking limit line (the line obtained by plotting the critical ignition timing against the air-fuel ratio) for regular gasoline, the air-fuel ratio corresponding to a given ignition timing is leaner than that on the knocking limit line for high octane gasoline and accordingly, if the ignition timing is controlled, along the knocking limit line, to the optimal value for regular gasoline, the air-fuel ratio will be inevitably controlled to a value leaner than the optimal value for high octane gasoline at which the exhaust gas temperature is on the higher limit, whereby the exhaust gas temperature is adversely raised. As is well known, the exhaust gas temperature substantially depends upon the air-fuel ratio and the ignition timing.
Though the problem described above may be overcome by changing the predetermined optimal value according the fuel octane value in light of the teaching in Japanese Unexamined Patent Publication Nos. 58(1983)-57072 and 58(1983)-143169. However, this approach is disadvantageous in that the octane number of gasoline is difficult to detect, it is almost impossible to prepare the optimal values for intermediate octane numbers obtained by mixing the regular gasoline and the high octane gasoline in various proportions, and the relation between the octane number and the knocking limit changes with time.